1. Field of the Invention
The present invention relates to an OFDM receiver using a polar coordinate system and method thereof, and more particularly to an OFDM receiver using a polar coordinate system and method thereof, which can simplify a calculation process therein. The present application is based on Korean Patent Application No. 2001-45293, which is incorporated herein by reference.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) method is a method that increases efficiency in frequency use by using a plurality of carriers having mutual orthogonal properties. The OFDM method, which uses multi-carriers in a wire or wireless channel, is adapted to use in a high-speed data transmission.
For example, in the wireless telecommunication channel having multi-path fading, when data having short symbol periods are transmitted at a high speed by a single carrier method, the interference between symbols is intensified and thereby the complicacy at the receiving end is increased. However, when the multi-carrier method is used, the symbol period at respective sub-carriers can be extended by as many as the number of the sub-carriers while the data transmission speed is maintained as high as ever. Therefore, a simple equalizer having one tap can process even the deep frequency selective fading by multi-paths.
FIG. 1 is a block diagram for illustrating a composition of conventional OFDM receiver 100.
Referring to FIG. 1, a received complex analog signal is converted into a digital signal at an analog-to digital (A/D) converter 110. The digital signal passed through the A/D converter 110 is multiplied by a correcting signal corresponding to estimated frequency offset to compensate a carrier wave frequency offset, at a synchronizer 120. To minimize an interference with other signals, a guard interval remover 130 removes a predetermined guard interval or band from the digital signal in which the frequency offset is compensated. After the guard interval is removed, the digital signal is input into a fast fourier transformer (FFT) 140. The FFT 140 carries out a fourier transform to the input digital signal. An equalizer 150 compensates a channel distortion to the digital signal outputted from the FFT 140. After compensating the channel distortion, a phase compensator 160 compensates a phase error remaining in the digital signal. Thereafter, a demapper 170 converts the digital signal into the most approximate value among constellations, which are used in the conventional receiver 100.
In FIG. 1, the received complex signal comprises an inphase (I) signal corresponding to a real number portion and a quadrature (Q) signal corresponding to an imaginary number portion, which are designated as solid lines, respectively. The complex signal has an orthogonal coordinate system until it is input into the demapper 170.
FIG. 2 is a block diagram for illustrating a composition of the synchronizer 120 using orthogonal coordinates, which is applied to the conventional OFDM receiver 100.
Referring to FIG. 2, the synchronizer 120 estimates a frequency offset by using estimation signals, and then outputs the estimated result as a complex value. The outputted complex value is converted into a phase through an arctangent function. The converted phase is converted into a trigonometric function, and multiplied by the received complex signal. Thus, a frequency offset is removed from the received complex signal by the synchronizer 120.
FIG. 3 is a block diagram for illustrating a composition of the equalizer 150 using the orthogonal coordinates, which is applied to the conventional OFDM receiver 100.
Referring to FIG. 3, the equalizer 150 receives estimation signals to estimate a channel distortion and multiplies a received complex signal by an estimated result to remove a distortion component in the channel.
FIG. 4 is a block diagram for illustrating a composition of the phase compensator 160 using the orthogonal coordinates, which is applied to the conventional OFDM receiver 100.
Referring to FIG. 4, a composition of the phase compensator 160 is similar to that of the synchronizer 120. That is, the phase compensator 160 estimates a phase error remaining in the received complex signal by using estimation signals, and outputs the estimated result as a complex value. The outputted complex value is converted into a phase through an arctangent function. The converted phase is converted into a trigonometric function, and then multiplied by the received complex signal. Thus, the phase error remaining in the received complex signal is compensated by the phase compensator 160.
To convert the phase of the complex signal, calculators for arctangent, sine and cosine functions are required to be disposed in the synchronizer 120, the equalizer 150, and the phase compensator 160. Particularly, estimators of components for transforming the phase such as the synchronizer 120, the equalizer 150 and the phase compensator 160 have a structure as described below.
First, a basic operation of each component is to estimate a value to be estimated by using a difference in phase between two input complex signals. Accordingly, to obtain the difference in phase between the two input complex signals input in the orthogonal coordinate system, it is necessary to use a conjugate complex multiplication according to the following mathematical expression 1.X*Y=(Xr+jXi)*(Yr+jYi)=(Xr−jXi)(Yr+jYi)=(XrYr+XiYi)+j(XrY1−XiYr)  [Mathematical Expression 1]
A phase, which is finally estimated according to the difference in phase obtained by the mathematical expression 1, can be obtained by the following mathematical expression 2.θ=tan−1(Im[X*Y]/Re[X*Y])=tan−1[(XrYi−X1Yr)/(XrYr+XiYi)]  [Mathematical Expression 2]
Accordingly, a phase of the complex signal can be compensated by multiplying the complex signal to be compensated by an estimated phase obtained by the mathematical expression 2, according to the following mathematical expression 3.Ze−jθ=(Zr+jZ1)(cos θ−j sin θ)=(Zr cos θ+Z1 sin θ)+j(Z1 cos θ−Zr sin θ)  [Mathematical Expression 3]
In the mathematical expression 1 and 2, the sign * means a conjugate complex multiplication.
Accordingly, as described with reference to FIGS. 1 through 4, it can be appreciated that in the conventional OFDM receiver, the complex multiplication is carried out two times at respective estimators in three components, so that the total complex multiplication for estimation and compensation is carried out six times.
Thus, since the conventional OFDM receiver processes the received signal in the orthogonal coordinate system, a system structure for the frequency synchronization, the compensation for channel influence, and the removal of remaining phase error as well as a calculation process therefor becomes complicated. As the system structure and the calculation process are complicated, the process time required in the frequency synchronization, the compensation for the channel influence, and the removal of remaining phase error is also lengthened. Particularly, the complicated calculation process may act as a factor which makes the received signal difficult to be correctly compensated.